1. Field of the Invention
The present invention relates to a protective circuit for controlling charging of a secondary battery and a battery charger including the protective circuit. The present invention also relates to a power storage device including a secondary battery in addition to the protective circuit or the battery charger.
2. Description of the Related Art
Overcharge and overdischarge of secondary batteries such as lithium ion batteries not only shorten their life but also cause ignition due to internal short-circuit. For this reason, a battery charger (also referred to as charger) for charging a secondary battery often includes a circuit for preventing overcharge and overdischarge, which is called a charging control circuit or a protective circuit, in addition to a power supply circuit.
The protective circuit recognizes the charging state of a secondary battery by monitoring a voltage across the secondary battery (a terminal voltage of the secondary battery). The protective circuit has a function of preventing the secondary battery from being overcharged in such a manner that, when the terminal voltage exceeds the upper limit, the protective circuit determines that the fully charged secondary battery is to be further charged, and interrupts the path of current supplied to the secondary battery. Alternatively, the protective circuit has a function of preventing the secondary battery from being over-discharged in such a manner that, when the terminal voltage falls below the lower limit, the protective circuit determines that the secondary battery is to be discharged beyond the capacity that should be left in the secondary battery, and interrupts the path of current discharged from the secondary battery.
Low power consumption is one of the important factors in evaluation of performance of electronic devices. In particular, low power consumption of a portable electronic device is strongly required because high power consumption leads to a disadvantage of short continuous use time. A secondary battery is often used in a portable electronic device such as a mobile phone; power necessary for the protective circuit to operate is supplied from the secondary battery when the secondary battery is not being charged. For this reason, lower power consumption of the protective circuit is also required.
Patent Document 1 discloses a protective circuit in which power consumption is reduced in such a manner that an overcharge detection circuit is turned on only for a short time at fixed intervals to perform overcharge detection operation. Patent Document 2 discloses a protective circuit in which power consumption is reduced by controlling the intervals between monitoring operations of an overcharge control circuit and an overdischarge detection circuit in accordance with a voltage across a secondary battery.